1. Field of the Invention
The present invention is directed to a frequency-selective and location-selective RF pulse sequence for a magnetic resonance apparatus and to a magnetic resonance tomography apparatus for generating such an RF pulse sequences.
2. Description of the Prior Art
In magnetic resonance tomography, the sharp resonant behavior of the nuclear magnetization of magnetic nuclei in biological tissue is utilized for generating an in vivo image of the human body with the assistance of a radio frequency field in the megahertz range and a location-dependent magnetic field. All atomic nuclei having an uneven atomic number have an intrinsic rotational axis, and therefore a nuclear magnetic moment. By far the greatest registerable sensitivity, however, exists for protons, i.e., the nuclei of hydrogen atoms. The exact nuclear resonant frequency (Larmor frequency) is dependent on the chemical environment of the respective proton. For example, the resonant frequency of hydrogen nuclei in free water compared to those participating in an aliphatic bond (fat) is shift by approximately 3 ppm (parts per million), i.e. approximately 130 Hertz given a field strength of 1.0 Tesla.
Since, in magnetic resonance tomography, the location information is encoded in frequencies on the basis of the magnetic gradient field, i.e. the nuclear magnetic resonant frequency changes along the gradient direction (slice normal), a spatial shift of the fat image part compared to the water image part occurs, which is represented as:
xcex94x =xcex94fxc3x97B0/Gr.
xcex94x Spacial shift due to the chemical shift
xcex94f Resonant frequency between water and fat protons
B0 Magnetic field strength
Gr Readout magnetic field gradient
The method for spatial resolution in magnetic resonance tomography is explained in H. Morneburg, xe2x80x9cBildgebende Systeme fxc3xcr die medizinische Diagnostikxe2x80x9d, Erlangen, 1995, Chapter 6.2, and the chemical shift is explained in Chapters 11.2.2 and 11.3.4.2.
The chemical shift between water and fat imaging is undesirable for many diagnostic applications. Methods for suppressing the chemical shift are therefore known wherein, for example, the fat signal is suppressed and only the water signal is presented. Such methods and apparatuses for the presentation of biological tissue with magnetic resonance tomography are disclosed, for example, in European Application 0 745 865 and U.S. Pat. No. 5,510,713 as well as German OS 38 10 018 and German OS 38 04 212. In the apparatuses and methods disclosed in these documents, a sequence of RF pulses having specific properties for nuclear magnetic excitation of a first medium, for example water, and for suppression of the excitation of a second medium, for example fat, is employed. Some of the known apparatuses and methods thus enable the selective presentation of a first medium, for example water, and of a second medium, for example fat, dependent on the desired application. German OS 35 43 854 discloses a nuclear magnetic resonance tomography method and an arrangement for the implementation of the method, wherein a pulse sequence of three RF pulses with bipolar magnetic field gradients is employed in order to generate images that only show the fat distribution, or images that only show the water distribution.
In another known method for suppressing the chemical shift, for example, a suppression of the fat signal and an exclusive presentation of the water signal are achieved by a pulse sequence of RF excitation pulses that are exactly matched in time, with which only the water protons are selectively excited, whereas nuclear magnetic excitation of fat protons is suppressed. The method is schematically shown In FIGS. 3 A and 3B herein, FIG. 3A shows the RF pulses and FIG. 3B shows the magnetic field gradient Gz in the slice normal. The first RF pulse at time t1 generates, for example, a rotation of the precessional angle of the nuclear magnetic moments by an angle xcex1/4. The second RF pulse at time t2 generates an angular change by xcex1/2, and the third pulse at time t3 in turn produces an angular change by xcex1/4, When the three RF pulses are emitted in phase, an overall angular modification by xcex1, for example 90xc2x0, occurs for the resonant nuclei. For selective excitation of only the water protons, the time interval t2xe2x88x92t1 of the first two pulses and t3xe2x88x92t2 of the second and third pulses is selected such that a phase shift of exactly 180xc2x0 occurs in this time period between the processing hydrogen protons and fat protons, The angular shifts of the fat protons generated by the individual RF pulses do not add, but subtract. The angular modifications in the water and fat protons therefore are as follows;
Water: 0xe2x86x92xcex1/4xe2x86x92xcex1/4xe2x86x923xcex1/4xe2x86x923xcex1/4xe2x86x92xcex1
Fat: 0xe2x86x92xcex1/4xe2x86x92xe2x88x92xcex1/4xe2x86x92xcex1/4xe2x86x92xe2x88x92xcex1/4xe2x86x920.
The first, third and fifth arrows respectively denote the angular change due to the first, second and third RF pulse and the second and fourth arrows indicate the angular change due to free precession.
An excitation of only the water protons thus is obtained. The fat protons generate no signal in a following measurement (readout). This suppression, however, is not complete, due to the reversed magnetic field gradients at the second RF pulse compared to the first and third RF pulses. The spatial region in which the phase relationship between the individual pulses is satisfied for both directions of the magnetic field gradient is very narrow at best. The fat suppression is therefore incomplete in edge regions lying next to this region at both sides thereof, and (disturbing) fat signals occur.
An object of the present invention is to provide a frequency-selective and location-selective RF pulse sequence for a magnetic resonance apparatus wherein a dependable suppression of the excitation of a second medium, for example fat, is achieved.
This object is achieved in a magnetic resonance imaging method in the form of a frequency-selective and location-selective RF pulse sequence with bipolar, selective magnetic field gradients for nuclear magnetic excitation of a first medium and suppression of the excitation of a second medium, wherein the frequency spectrum of the pulses is selected such that the spatial resonance range of pulses for the second medium, given a first polarity of the magnetic field gradient, essentially coincides with the spatial resonant region of the RF pulses having opposite polarity of the magnetic field gradient. Due to the frequency compensation for the medium to be suppressed, a nearly complete suppression of the signals from this medium, for example fat, can be achieved. The attenuation of the signal from the first medium which occurs as a side effect is therefore of little consequence compared to the image quality that can be achieved.
The pulse sequence preferably includes three pulses that are equidistantly spaced in time, whereby the two outer pulses effect a rotation of the nuclear magnetization of the first and second media by a first angle and the middle pulse effects a rotation of the nuclear magnetization of the first medium by twice the amount of the first angle in the same direction, and effects a rotation of the nuclear magnetization of the second medium by twice the angle in the opposite direction. The selection of the angles can be made at the discretion of a person skilled in the art corresponding to the particular application.
The spacing between the pulses is preferably selected such that it corresponds to a phase difference of 180xc2x0 between the precessing nuclear magnetization vectors of the two media.
The invention is also directed to a magnetic resonance tomography apparatus having an RF unit for generating RF pulses and a magnet arrangement for generating a constant magnetic field and a magnetic field gradient superimposed thereon, wherein the RF unit operates to generate the frequency-selective and location-selective RF pulse sequence described above.